Instrumentation circuit with direct
coupled amplifier having tempera-
ture stabilization

ABSTRACT

13. A MONITORING CIRCUIT COMPRISING THE COMBINATION OF A DIRECT COUPLED AMPLIFIER HAVING OUTPUT CHARACTERISTICS WHICH VARY WITH TEMPERATURE, A CONDITION RESPONSIVE SIGNAL INPUT DIRECT COUPLED TO SAID AMPLIFIER, MEANS FOR COMPENSATING FOR A PREDETERMINED CHARACTERISTIC OF SAID CONDITION RESPONSIVE SIGNAL INPUT, SAID MEANS HAVING A TEMPERATURE DEPENDENT OUTPUT AND BEING DIRECT COUPLED TO SAID AMPLIFIER AND MEANS RESPONSIVE TO TEMPERATURE CAUSED CHANGES IN SAID AMPLIFIER AND SAID MEANS FOR COMPENSATING FOR A PREDETERMINED CHARACTERISTIC FOR GENERATING A COMPENSATING SIGNAL TO SIMULTANEOUSLY COMPENSATE SAID AMPLIFIER AND SAID MEANS FOR COMPENSATING FOR PREDETERMINED CHARACTERISTIC.

0d. l, 1914 F E, BECK EVAL Re. 28,184

II'STRUIENTATION CIRCUIT WITH DIRECT CDUPLED AIPLIFIER HAVINGTEMPERATURE STABILIZATION Original Filed July 19. 1965 UnitedStatesPatent O 28,184 INSTRUMENTATION CIRCUIT WITH DIRECT CUUPLED AMPLIFIERHAVING TEMPERA- TURE STABILIZATION Frederic E. Beck, Rochester, and EliL. Garelick, Henrigt? NX., assignors to Transmation, Inc., Rochester,Original No. 3,486,127, dated Dec. 23, 1969, Ser. No. 472,899, July 19,1965. Application for reissue Dec. 14, 1971, Ser. No. 207,755

Int. Cl. H031? 3/68 U.S. Cl. 330-30 D 38 Claims Matter enclosed in heavybrackets appears in the original patent but forms no part of thisreissue specification: matter printed in italics indicates the additionsmade by reissue.

ABSTRACT F THE DISCLOSURE The disclosure relates to an instrumentationcircuit using a [D-C] direc! coupled amplifier `without a chopper whichhas relatively minimal noise and drift, this being provided by the useof a circuit which corrects for temperature drift and common modeimpedance and which can also perform accurately when measuringthermocouple response. The circuit also has provision for offset andthereby amplify the scale of a particular small range of interest.

This invention relates to electronic apparatus and more particularly toan electrical instrumentation circuit.

Now that many industrial processes are becoming automated at an everincreasing pace, a great demand has arisen for extremely accurate,highly flexible sub-systems which can monitor process conditions such aspressure, temperature, liquid level, etc. at remote locations andproduce an electrical signal which is accurately representative of theseconditions. This signal may then be used for a number of differentpurposes, for example, it may be employed to trigger an alarm whensignal reaches a prearranged level or it may be transmitted to anotherpart of the system which controls the process being monitored, or infact, the signal may be employed for both purposes. Although this typeof circuit may in some instances be employed in very simple systemswhere it is used to sound an alarm and/or shut down a piece of equipmentwhen temperature or pressure becomes excessive, the equipment which itis being employed to protect and control is generally very expensivecapital equipment which is frequently valued in the hundreds ofthousands of dollars and accordingly the circuit must be extremelydependable and preferably very [log] long lived so as to prevent theshut down of this capital equipment for the replacement of theprotective circuit. As the complexity of the total system in which thesemonitoring circuits are employed grows, the requirement for highdependability in them becomes even more stringent. Thus, for example, asmany industries automate their manufacturing processes they frequentlyemploy fairly large numbers of these monitoring circuits in the feedback loop of an overall process control system and Where a number ofvery complex and interrelated factors which are continuously changinghave some bearing on the outcome of the process, many of thesemonitoring circuits are frequently connected as inputs to a computerwhich reads the monitor outputs and then modifies process conditionsaccordingly so as to optimize process results. Considering theinterrelated operation of the many components in this type of' system,the large number of monitoring system frequently used and the expense ofshutting down and starting up large, continuous manufacturing processesthe requirements for dependability, ease of maintenance, low cost andflexibility become even more apparent.

Ressued Oct. 1, 1974 .'llCe In the past, monitoring circuits of the typedescribed have generally employed vacuum tube circuits of the mechanicalchopper stabilized type. In one type of chopper stabilized prior artmonitoring circuit the DC input was chopped to AC, amplified and thendemodulated while in another type the input was direct coupled, theerror was chopped and a correcting term was then added so that an ACamplifier would correct the drift of the primary DC amplifier. Thoughone of these two types of chopper stabilized amplifier was generallyemployed in the prior art in order to achieve adequate stability andaccuracy, the use of choppers in these circuits makes them relativelyexpensive and short lived, while at the same time introducing afrequency response problem and thereby limiting their use to arelatively narrow range so that different units must be designed tocover various ranges. Accordingly, in order to install a completecontrol system in a continuous manufacturing process such as oilrefining their short life and relative lack of reliability is multipliedby the number of units which must be installed and maintenance problemsbecome extremely difiicult, time consuming and expensive. In addition,many prior art monitoring circuits suffer from a further lack offlexibility in that they are unable to accept more than One type ofinput. For example, while one of these circuits may be employed with avariable resistance input such as a strain gage it cannot also be usedto monitor a voltage generating input such as a thermocouple. Relativelylow input impedance is also a problem with many prior art monitoringcircuits.

Accordingly, it is an object of this invention to provide a monitoringcircuit which is devoid of the aforementioned deficiencies.

It is further object of this invention to provide an extremely accuratemonitoring circuit.

It is also an object of this invention to provide a monitoring circuitwith high input impedance.

It is a further object of this invention to provide a monitoring circuitwhich can accept many different types of inputs.

It is further object of this invention to provide a monitoring circuitof novel design which is both inexpensive to produce and highlydependable.

Yet another object of this invention is to provide a universal outputmonitoring circuit.

An additional object of this invention is to provide a monitoringcircuit whose output can be transmitted to a controlled computer withoutthe necessity of an interface circuit.

It is also an object of the invention to provide a monitoring circuitwhose output is extremely stable with varying load resistance.

Yet another object of the invention is to provide a monitoring circuitwhose output is extremely stable with changes in ambient temperature andline voltage.

The above and still further objects of the invention are accomplished,generally speaking, by providing a solid state monitoring circuitemploying a direct coupled amplifier circuit with a signal generatorwhich exactly compensates for changes in the level of the referencesignal and characteristics of the amplifier induced by temperaturechange in the circuit. The circuit preferably employs a matched pair oftransistors in a differential amplifier with a temperature responsivecircuit which provides extremely fine compensation for the referencevoltage and for small differences in the responses of the twotransistors in the differential amplifier as they track the voltageversus, ambient temperature curve. In effect then, the compensatingcircuit converts the matched transistor pair to a super matched pairwhile also rendering the net reference signal output insensitive totemperature changes.

When a differential amplifier is employed, a temperature compensatedconstant current source is preferably also used to aid in stabilizingthe amplifier and provide an unchanging offset output signal. Otherfeatures of the circuit include a provision for either a variablevoltage generating input or a variable resistance input, output spanadjustment with adjustable [O] zero and null balance circuitry withcurrent feedback to provide a true current output.

The invention will be more clearly understood after reading thefollowing detailed description of an exemplary embodiment thereof andespecially when this is read in conjunction with the accompanyingdrawings wherein:

The figure is a schematic circuit diagram illustrating an exemplaryembodiment of the invention.

Referring now to the drawing it is seen that a conventional 117 volt ACpower source 11 is connected to the primary winding of a transformer 12,the secondary winding of which is connected to a full wave dioderectifier 13.

A capacitor 14 shunts the output of the full wave rectifier to smooththe output wave and remove excessive ripple therefrom. The output ofthis power supply is also applied across a current limiting resistor 16connected in series with a 20 volt Zener diode 17. The 2O volt outputacross Zener diode 17 is applied across a second current limitingresistor 18 connected in series with two 7.5 volts Zener diodes 19 and21. The two diodes are employed here to provide 7.5 and volt sources. Itis of course, to be understood that these elements constitute the powersupply for the system and accordingly they may be replaced by anysuitable equivalent capable of providing the required power. lt is alsoto be understood, that all of the values herein for voltages, currents,resistances, etc. in this power supply circuit and throughout theremainder of the circuit are exemplary only, and may be varied withvariations in other component changes in input or output requirements,etc. It also should be understood, that many of the solid statecomponents in this circuit such as the diodes in the full Waverectifier, may be replaced by their vacuum tube or other equivalentswhile still coming within the scope of the invention. In addition to itsuse as an initial reference stage to feed the l5 volt Zener diodevoltage regulator pair, the 20 volt source is used to feed thedifferential amplifier and the tirst gain stage as brought out morefully hereinafter. Connected in series across the 15 volt line are twonominal 499 ohm resistors 22 and 23 and a ten ohm potentiometer 24. Twomatched temperature sensitive resistors 26 and 27 are connected inseries across potentiometer 24 and a point between resistor 26 and 27 isconnected above Zener diode 21 so that a 7.5 volt potential is appliedto this point in the bridge. Any suitable temperature sensitiveresistors such as wound copper may be employed as resistors 26 and 27.The wiper of potentiometer 24 is connected to a reference or coldjunction thermocouple 29 which in turn is connected in series with themonitoring thermocouple 30. It should be noted that referencethermocouple 29 is connected to buck monitoring thermocouple 30.Resistors 26 and 27 have a positive temperature coefficient, however twonegatve coefficient elements may be used with cqual results. One of thefunctions of these resistors is to introduce a signal which is of equalmagnitude but opposite in direction from the signal error caused by thechange in temperature of the reference thermocouple.

The compensating circuit is also employed to compensate for temperaturechange induced signal errors in the matched pair of transistors in thefirst stage of the differential amplifier, because no matter howcarefully these transistor pairs are matched they generally track theoutput voltage versus temperature plot along different lines. Sinceeither one of the matched pair of transistors 31 and 32 may tend toproduce higher voltage outputs with increasing temperature than theother and the direction of signal error in the completed circuit isunpredictable, the temperature compensating portion of the circuit islai-directional. Accordingly, if the wiper on the ten ohm potentiometeris set at the exact center of its range and resistors 26 and 27 areexactly equal in value then the temperature coefficient introduced bythe network is 0. If however, the ten ohm resistor is moved in adirection such as to short out a portion of one or the other of theresistors then either a positive or negative voltage is introduced withtemperature. This voltage appears in series with the referencethermocouple and acts as an additional signal input. After the circuitis completely built and tested at the various temperatures which it willencounter in use, the potentiometer 24 is set so as to compensate forboth the signal error introduced by changes in temperature of thereference thermocouple and for those introduced by changes intemperature of matched pair 31-32. So long as reference thermocouple 29,temperature, compensating resistors 26 and 27 and the matched transistorpair 31-32 are all included together within a fairly small case so thatthey all undergo approximately the same changes in temperature,satisfactory compensation will be introduced. However, for best results,these elements should be placed as close together physically as possiblein the unit and for Optimum performance they may be encapsulatedtogether, as for example, in an epoxy resin so that they will all beretained at almost exactly the same temperature when the device is inoperation. The lead of monitoring thermocouple 30, most remote fromreference thermocouple 29 is connected to the base of transistor 32which in conjunction with transistor 31 forms the first stage of adirect coupled diiferential amplifier. A Zener diode 34, a diode 36 anda 10K resistor 37 are all connected in series across the fifteen voltregulated supply provided by the two Zeners 19 and 21. The base of atransistor 38 is connected to a point between diode 36 and resistor 37while the emitter of transistor 38 is connected to the base of the 15volt line through a 25K resistor 39 and a temperature sensitive resistor4l. Resistor 41 is also shunted with a variable resistance 42. Thecollector of transistor 38 is connected to the wiper of potentiometer 43through a 100 ohm resistor 44. The potentiometer 43 is in series withthe emitters of transistors 31 and 32. A 499 ohm resistor 46 and a 1Kresistor 47 are also provided as open circuit shunts across thepotentiometer so that by closing either one or both of these shunts therange of the potentiometer may be widely varied. Elements 34 through 47provide a temperature compensated constant current source for thedifferential amplifier so that the necessary degree of stability isprovided to allow for direct coupling of the differential amplifier.Zener 34 is provided primarily for close regulation of the base toemitter supply voltage across transistor 38 so as to stabilize itscurrent output. Since the base to emitter voltage across transistor 38has a relatively large temperature coefficient while the Zener 34 hasonly a small positive temperature coefficient, diode 36 in series withthe Zener is provided because it has a temperature coefficient which isessentially equal to that of the base to emitter coefficient oftransistor 38. In order to provide exactly balanced temperaturestability, temperature sensitive resistor 41 is provided with a positivetemperature coetiicient and is shunted with potentiometer 42. Thisconstant current source may be precisely balanced by measuring thevoltage across the output resistor 44 with changes in ambienttemperature and adjusting the wiper on potentiometer 42 until there isno change in voltage across resistor 44 with changes in temperature. Anysuitable temperature sensitive resistor may be used as element 41.Potentiometer 43 is employed for zero adjustment. That is to say, it isemployed to unbalance the differential amplifier so as to provide apreselected output level for a zero input. It is generally desirable inthe control industry that a preselected output such as a 4 milliamps beprovided even for a zero input as an indication that the monitoringcircuit is operative. By leaving open or closing either one or both ofshunting resistors 46 and 47, zero adjusting potentiometer 43 may bereadily employed to adjust the output current provided with zero inputto one milliamp, four milliamps, ten milliamps, etc. Other shunts mayalso be provided to extend the range, improve the resolution orotherwise change this adjustment.

A large capacitor 48 is connected from the input lead of the base oftransistor 32 to common so as to filter out any ripple on the inputsignal as it is fed into the transistor base. Also applied to the baseof a transistor 32 is a signal from resistor 49 which is tied to the 15volt supply so as to provide the initial offset base current for thecollector current of that transistor. The collector output oftransistors 31 and 32 are connected to the bases of the second stagetransistors 53 and 54 and are also connected to the 20 volt lineregulated by Zener 17 through two 100K resistors 56 and 57 which loadthe first stage. The common emitters of transistors 53 and 54 areconnected to the common base line through a 220K transistor 58. Thecollector of transistor 54 is also connected to the 20 volt line througha 100K resistor 59. The output of this collector drives transistor 61, aPNP stage which provides considerable voltage gain. The collector ofthis stage is connected to the base of transistor 66 and also to a 4.7mfd. capacitor 63 and a 22K resistor 64 which forms a filter network tofurther reduce noise and any AC response which might have beenintroduced into the system and at the same time prevent the system fromoscillating. Transistor 66 forms the output stage which supplies theoutput current through [fedeback] feedback resistors 67 and 68. Feedbackresistor 68 is a potentiometer so that a portion of the feedback signalmay be selected by manipulation of the wiper on the potentiometer. Thepotentiometer itself may be shunted by anyone or more of three optionalshunting resistors 71, 72 or 73 of 15, 3 and 1.5 ohms respectively so asto enable setting of the output span range. Thus by merely shunting withthe proper resistors here any one of the standard output span ranges canbe provided. In effect then, the output span range is controlled bycontrolling the amount of feedback in the circuit. The wiper ofpotentiometer 68 is connected to the base of transistor 31 through a tenohm resistor 74 to complete the feedback loop. A large value resistor 76connects the 7.5 volt base line to the input of the base of transistor32 of the difference amplifier so as to slowly drain filter capacitor 48to prevent it from charging to the full volt level on open circuit.

When signal generating inputs (generally in the millivolt range) otherthan thermocouple 30 are employed the reference thermocouple 29 is alsoremoved from the circuit or shorted and the circuit may then bereadjusted for temperature compensation according to the proceduredescribed above.

It is also to be noted, that this circuit may be employed to monitorelectrical elements which do not generate a signal of their own and anysuitable condition responsive, variable impedance may be monitored bythe circuit, such as temperature sensitive resistors. Thus, for example,in monitoring the variation in resistance with temperature change ofsuch a resistor the thermocouple 30 and reference thermocouple 29 areremoved from the circuit and the temperature sensitive resistance isconnected in series with a resistor between 7.5 volts and l5 volt lines.[The connection of this resistors is connected] The junction of thesetwo resistors s connected to the end of resistor 33 which is most remotefrom the base of transistor 32. In this way the power supply of thecircuit itsclf applies a voltage across the variable impedance beingmonitored and the current How through the impedance is then proportionalto the temperature being monitored. In other words, the circuit has abuilt in power supply for any element being monitored which does notitself generate a voltage.

The output of the circuit described is taken across terminals 78 and 79with terminal 78 being connected to one side of the full wave rectifier13 through a power limiting resistor 77 which prevents over loading ofthe circuit. The other terminal 79 of the output is connected to thecollector of transistor 66.

Regardless of whether the circuit is used to monitor a thermocouple,another signal generating input or condition responsive impedance, thesignal produced is compared against a reference signal of one type oranother. This reference may consist of the output of a referencethermocouple, an offset voltage or some other source. In addition tothese sources, other thermocouple sources are generally formed in thecircuit inadvertently at solder connections or other junctions thusintroducing in all cases at east a reference signal error caused byternperature change of these junctions. The adjustable compensatingcircuit accordingly corrects for input signal error caused bytemperature change of the reference voltage source and, the inadvertentthermocouples as well as for temperature induced changes in theamplifier characteristics and this compensation of compensation in theconstant current source for the amplifier which is separately providedso compensation is accurately provided over a wide range of ambienttemperatures, without interaction.

It is of course to be understood, that the specific circuit describedabove is only illustrative of the invention and that many other changes,modifications, additions and the like which still come within the scopeand spirit of the invention will be obvious to those skilled in the artupon review of the specification. It is of course intended that these beincluded within the invention.

When used throughout the specification and claims the phase conditionresponsive voltage generating input is to be taken to include not onlythose elements which generate a voltage internally in response tochanges in ambient conditions but also those which change theirirnpedance and are connected across the power supply of the monitoringcircuit.

What is claimed is:

[1. A monitoring circuit comprising a D-C amplifier, a conditionresponsive signal input and a feedback voltage generating input directlycoupled to said amplifier, said feedback voltage generating input andsaid amplifier having output characteristics which vary withternperature, and a compensating voltage generating means directlycoupled to said amplifier, said compensating voltage generating meanshaving an output which varies with temperature in such a way as tooffset the net change in the output of said amplifier caused byvariations in the characteristics of said amplifier and the output ofsaid feedback voltage generating input induced by variations intemperature] [2. A monitoring circuit according to claim 1 in which saidcondition responsive voltage generating input is a thermocouple] [3. Amonitoring circuit according to claim 1 in which said compensatingvoltage generating means is bi-directionaly variable] .[4. A monitoringcircuit according to claim 1 further including a power supply and inwhich said condition responsive voltage generating input comprises acondition responsive variable impedance connected across an output fromsaid power supply] [5. A monitoring circuit according to claim 1 inwhich said amplifier is a differential amplifier] [6. A monitoringcircuit according to claim 5 further including a temperature compensatedconstant current source for said differential amplifier] [7 A monitoringcircuit according to claim 6 in which said constant current sourcecomprises a transistor with its emitter connected in series with atemperature sensitive resistor and a closely regulated input voltageconnected across said resistor and the base of said transistor] [8. Amonitoring circuit according to claim 6 in which said constant currentsource comprises a transistor with its emitter connected in series witha temperature sensitive resistor a potential source connected acrosssaid resistor and the base of said transistor, a variable resistorshunting said temperature sensitive resistor with the voltage variationof said temperature sensitive resistor and the base to emitter voltagevariation of said transistor being larger in sum than that of saidpotential source whereby exact temperature compensation may be achievedin said constant current source by adjustment of said variable shuntingresistor] [9. A monitoring circuit according to claim 1 in which saidfeedback voltage generating input and amplifier and said compensatingvoltage generating means are physically adjacent to each other] [10. Amonitoring circuit according to claim 2 in which said thermocouple isremote from said monitoring circuit and further including a referencesignal input in said monitoring circuit summed with said signal input tobuck the output of said signal input] 11. A monitoring circuit for acondition responsive voltage generating input, at least one conditionresponsive element of which is apart from said monitoring circuitcomprising a differential amplifier directly coupled to said input andto a reference signal generating input, a compensating voltagegenerating input directly coupled with said amplifier, said compensatinginput containing a bidirectionally adjustable temperature sensitivenetwork so that the output voltage which it generates upon changes intemperature can be varied to compensate exactly for temperature inducedvariations in said reference signal and temperature induced mismatch insaid differential amplifier and a temperature compensated constantcurrent source for said diterential amplifier, said constant currentsource comprising a transistor with its emitter connected in series witha temperature sensitive resistor, a potential source connected `acrosssaid resistor and the base of' said transistor, a variable resistorshunting said temperature sensitive resistor with the voltage variationof said temperature sensitive resistor and the base to emitter variationof said transistor being larger in sum than that of said potentialsource whereby exact temperature compensation may be achieved in saidconstant source by adjustment of said variable shunting resistor.

12. A monitoring circuit for a remote thermocouple comprising areference thermocouple connected to buck the output of said remotethermocouple, the combined output of said two thermocouples beingdirectly coupled to a differential amplifier with a temperaturecompensated constant current source, and a second voltage generatinginput directly coupled to said amplifier, said second input containing abi-directiorlally adjustable temperature sensitive network so that theoutput voltage which it generates upon changes in temperature can bevaried, said reference thermocouple, the temperature sensitive[resistors in said bridge] network and said differential am plifierbeing closely adjacent each other so that said network may be adjustedto compensate exactly for changes in the output of said referencethermocouple and mismatch in said amplifier induced by changes inambient temperature of said monitoring circuit, said constant currentsource comprising a transistor with its emitter connected in series witha temperature sensitive resistor, a potential source connected acrosssaid resistor and the base of said transistor, a variable resistorshunting said temperature sensitive resistor with the voltage variationof said temperature sensitive resistor and the base to emitter voltagevariation of said transistor being larger in sum than that of saidpotential source whereby exact temperature compensation may be achievedin said constant current source by adjustment of said variable shuntingresistor.

13. A monitoring circuit comprising the combination of o direct coupledamplifier having output characteristics which vary with temperature, acondition responsive signal input direct coupled to said ampliher, meansfor compensating for u predetermined characteristic of said conditionresponsive signal input, said means haring a temperature dependentoutput and being direct coupled to said amplifier and means responsiveto temperature caused changes in .raid ampliher und said means forcoinpensating for a predetermined characteristic for generating acompensating signal to simultaneously compensate said unlplijer and saidmeans for compensating for prcdetermined characteristic.

i4. A monitoring circuit as set forth in claim. I3, said amplier furtherincluding un input and output und feedback means coupled between theoutput of Suid umplier und the input thereof.

l5. A monitoring circuit as set forth in claim 14 further includingmeans in said feedback means for pr0- riding dierent output rouges.

I6. A monitoring circuit as set forth in claim I3 said ampli/fierfurther including means at the output thereof for providing a singleended high output resistance for providing a current output unaffectedby loud variations.

I7. A monitoring circuit as set forth in claim 14 said amplifier furtherincluding means at the output thereof for providing a single ended highoutput resistance for providing a current output unu'ecled by loadvariations.

18. A monitoring circuit according to claim I3 in which saidcompensating voltage generating means is bidirectionally variable.

19. A monitoring circuit according to claim I4 in which saidcompensating voltage generating means is bidirectionally variable.

2t). A monitoring circuit according to claim 13 in which said amplifierincludes n differential amplifier.

2]. A monitoring circuit according to claim 14 in which said amplifierincludes a differential amplier.

22. A monitoring circuit according to claim I9 in which said amplifierincludes a dierentiul ampliher.

23. A monitoring circuit according t0 claim I3 in which said means forcompensating said amplifier and said compensating signal generatingmeans are thermally coupled to each other.

24. A monitoring circuit according to claim 14 in which said means forcompensating said amplifier and said compensating signal generatingmeans are thermally coupled to each other.

25. A monitoring circuit according to claim 22 in which .raid means forcompensating said mnplier and .mid compensating signal generating meansare thermally coupled to each other.

26. A monitoring circuit as set forth in claim 2l wherein saiddifferential amplifier has a derentiul input und a `single ended output.

27. A monitoring circuit as set forth in claim 22 wherein saiddijerential amplifier has a differential input and a single endedoutput.

28. A monitoring circuit comprising the combination of a direct coupledamplifier having output characteristics which Vary with temperature, athermocouple input direct coupled to said ampliher, u cold junctionhaving a temperature dependent output and being direct coupled to theinput of said umpliher and means responsive to temperature causedchanges in said amplifier and said cold junction for generating acompensating signal to simultaneously compensate said amplifier and saidcold junction.

29. A monitoring circuit as .ret forth in claim 28, .mid ampli/ierfurther including un input and output and feedback means coupled betweenthe output of said amplier and the input thereof.

30. A monitoring circuit as set forth in claim 29 yfurther includingmeans in said feedback means for providing different output ranges.

31. A monitoring circuit as set forth in claim 28 said amplifier furtherincluding means at the output thereof for providing a single ended highoutput resistance for providing a current output unaected by loadvariations.

32. A monitoring circuit as set forth in claim 29 said amplifier furtherincluding means at the output thereof for providing a single ended highoutput resistance for providing a current output unaffected by loadvariations.

33. A monitoring circuit according to claim 28 in which saidcompensating voltage generating means is bi-directionally variable.

34. A monitoring circuit according to claim 29 in which saidcompensating voltage generating means is bidirectionally variable.

35 A monitoring circuit according to claim 28 in which said amplierincludes a differential amplifier.

36. A monitoring circuit according to claim 29 in which said amplifierincludes a differential amplifier.

37. A monitoring circuit according to claim 34 in which said amplifierincludes a differential amplier.

38. A monitoring circuit according to claim 28 in which said means forcompensating said amplifier and said compensating signal generatingmeans are thermally coupled to each other.

39. A monitoring circuit according to claim 29 in which said means forcompensating said amplifier and said compensating signal generatingmeans are thermally coupled to each other.

40. A monitoring circuit according to claim 37 in which said means forcompensating said amplifier and said compensating signal generatingmeans are thermally coupled to each other.

41. A monitoring circuit as set forth in claim 36 wherein saiddierential ampliyer has a differential input and a single ended output.

42. A monitoring circuit as set forth in claim 37 wherein saiddierential amplifier has a dierential input and a single ended output.

43. A monitoring circuit according to claim 20 further including atemperature compensated constant current source for said dieren tialamplifier.

44. A monitoring circuit according to claim 43 in which said constantcurrent source comprises a transistor with its emitter connected inseries with a temperature sensi-- tive resistor and a closely regulatedinput voltage connected across said resistor and the base of saidtransistor.

45. A monitoring circuit according to claim 44 in which said constantcurrent source comprises a transistor with its emitter connected inseries with a temperature sensitive resistor a potential sourceconnected across said resistor and the base of said transistor, avariable resistor shunting said temperature sensitive resistor with thevoltage variation of said temperature sensitive resistor and the base toemitter voltage variation of said transistor being larger in sum thanthat of said potential source whereby exact temperature compensation maybe achieved in said constant current source by adjustment of saidvariable shunting resistor.

46. A monitoring circuit according to claim 35 further including atemperature compensated constant current source for said differentialamplifier.

47. A monitoring circuit according to claim 46 in which said constantcurrent source comprises a transistor with its emitter connected inseries with a temperature sensitive resistor and a closely regulatedinput voltage connected across said resistor and the base of saidtransistor.

48. A monitoring circuit according to claim 47 in which said constantcurrent source comprises a transistor with its emitter connected inseries with a temperature sensitive resistor a potential sourceconnected across said resistor and the base of said transistor, avariable resistor shunting said temperature sensitive resistor with thevoltage variation of said transistor being larger in sum than that ofsaid potential source whereby exact temperature compensation may beachieved in said constant current source by adjustment of said variableshunting resistor.

References Cited The following references, cited by the Examiner, are 0frecord in the patented le of this patent or the original patent.

UNITED STATES PATENTS 3,046,487 7/1962 Matzen et al 330-69 X 3,283,57911/1966 Josephs 73-359 3,290,520 12/1966 Wennik S30-69 X 3,305,7342/1967 Buitenho" S30- 30 X HERMAN KARL SAALBACH, Primary Examiner L. Ll.DAHL, Assistant Examiner U.S. C1. X.R` B30-23, 28, 30 R, 69

